For exercise I recommend 3 days of cardio such as running, biking, fitness walking, running, etc and 3 days of strength training to build more muscle mass as this will enable your body to be an efficient fat burning machine. As for training your abs, you can train them no more than 3 days per week. You can do your ab workouts either on your cardio days or strength training days, but please don't spend a lot of time here. I get all the ab training I need in 10 minutes or less. I focus on variety and working my entire core which includes my lower back. If you have lots of fat to burn, your time is better spent on doing more cardio.